1. Field of the Invention
This invention relates to an electrically data rewritable non-volatile semiconductor memory device. More particularly, it relates to a multi-value flash memory adapted to store a multi-valued data in a memory cell.
2. Description of the Related Art
In a flash memory, the accumulated electric charge of the floating gate of a memory cell transistor is changed as the stored data is erased and a new data is written there. Then, as a result, the threshold voltage is changed to store the data. For instance, the negative threshold voltage may be made to corresponds to a “1” data, whereas the positive threshold voltage may be made to corresponds to a “0” data.
In recent years, multi-value flash memories adapted to store a plurality of bits in a single memory cell have been developed to reduce the cost per bit and/or increase the storage capacity. In a memory device adapted to store two bits in a single memory cell, the memory cell has four threshold voltages depending on the data to be stored there.
A highly reliable memory device can be obtained by accurately controlling the threshold voltages of each memory cell. “Fast and Accurate Programming Method for Multi-level NAND EEPROMs, pp. 129-130, Digest of 1995 Symposium on VLSI Technology” proposes a method of writing data, raising the write voltage Vpgm at a rate, in order to precisely control the threshold voltages of each memory cell.
With the method proposed in the above cited document, the width of distribution of each threshold voltage can be controlled theoretically to as small as 0.2V by raising the write voltage Vpgm at a rate of 0.2V/10 μsec. Normally, the write voltage Vpgm is divided into a plurality of write pulses and the voltage Vpgm of the pulses is raised stepwise at a predetermined rate. This technique provides an effect similar to that of continuously raising the write voltage Vpgm. The threshold voltage is checked after applying each pulse to the memory cell and the write operation is terminated when the threshold voltage has got to a predetermined verification level.
Meanwhile, micronization of processing dimensions is in progress. This means that the gaps separating memory cells are made smaller and smaller to consequently give rise to various problems from the viewpoint of multi-valued flash memories. For instance, the distance separating floating gates is reduced to produce problems as pointed out below as a result of micronization.
Imagine two memory cells A and B arranged side by side. Assume that the data of the two memory cells are erased simultaneously and they are made to have a threshold voltage of −3V. Then, firstly a data is written into the memory cell A. As a result, its threshold voltage may be raised to 0.5V to 1V. Subsequently, another data that is different from the data written into the memory cell A is written into the memory cell B. As the threshold voltage of the memory cell B is raised to 1.5V to 2V, the electric potential of the floating gate of the memory cell A falls and its threshold voltage is raised, say, to 1V to 1.5V as a result of the capacitive coupling of the floating gates of the two memory cells.
In the above described instance, the difference of the threshold voltages of the memory cells A and B (read out margin) should be at least 0.5V. However, it is reduced to 0V as a result of the capacitive coupling of the floating gates of the two memory cells. Thus, the difference of the threshold voltages necessary for discriminating two different data is reduced and the read out margin disappears.
It may be conceivable to reduce the stepwise increment Dvpgm of the write voltage Vpgm in order to avoid this problem. For example, the distribution width of the threshold voltage is reduced from 0.5V to 0.1V to increase the write out margin by 0.4V by reducing the stepwise increment Dvpgm from 0.5V to 0.1V.
However, as the stepwise increment is reduced to ⅕ of the original value, the number of pulses becomes five times as many as the original number. Then, the write time will become five times as long as the original value to give rise to a new problem.
For example, Japanese Patent Application KOKAI Publication No. 2003-196988 discloses a technique of reducing the difference of the threshold voltages without reducing the stepwise increment Dvpgm of the write voltage Vpgm. With the disclosed technique, a write operation is conducted by supplying the write voltage that is stepwise incremented by Dvpgm and the write control voltage of 0V being applied to the bit line to the memory cell. When the memory cell approaches a predetermined write state, the write control voltage is raised from 0V to, for example, 0.4V to reduce the rate at which the threshold voltage is changed and the write operation to the memory cell is terminated while the rate of changing the threshold voltage is reduced.
As discussed above, with the technique disclosed in Japanese Patent Application KOKAI Publication No. 2003-196988, the write control voltage is raised during a write operation in order to reduce the rate of changing the threshold voltage. However, the rate of changing the threshold voltage returns to the original level sooner or later because the write voltage itself rises stepwise. Therefore, it is necessary to sufficiently reduce the rate of changing the threshold voltage and hence it takes time from the time when the rate of changing the threshold voltage is reduced to the time when the write operation is completed. Additionally, the performance of controlling the threshold voltage is not necessarily satisfactory.
Therefore, so far, any attempt at securing a write out margin and raising the reliability of a memory device is accompanied by the problem of an increased write time.